The present invention relates generally to force and strain sensors; more particularly to a system and method of measuring static load using the piezoelectric effect with a feedback technique to compensate for signal loss.
Static load measurement is widely used in industrial and commercial applications, from machine tools to transportation systems (e.g., trains, forklifts, trucks, cranes) to scales for postal, freight, or personal weighting purposes.
Various physical principles have been explored over the past few decades for static load measurement, resulting in sensors that either make use of simple structural deflection of certain mechanical elements (e.g., springs and balances) or are based on complex physical transduction mechanisms (e.g., magnetoelastic or optoelastic). In engineering practice, the most widely used, state-of-the-art sensors for static load measurement are the strain gauges.
A strain gauge functions as a resistive elastic sensor, the resistance of which varies as a function of applied strain. In other words, strain gauges relate to the deformation experienced by the strain gauge sensing elements (e.g. metallic wire or semiconductors) attached to a load-carrying structure to the mechanical loads applied to the structure, through changes in electrical resistance of the sensing elements. The advantage of a strain gauge-based sensing technique is that it can measure pure static load of low variation frequencies (down to the DC range) and provide a stable output in the form of an electronic voltage signal.
Strain gauge-based load measurement, however, has a number of drawbacks. For example, strain gauges need to be constantly powered, requiring a power source (e.g., by means of batteries or an external power supply). Typically, strain gauges are connected into a bridge circuit, for example a Wheatstone bridge circuit, in order to be functional. Such a requirement severely limits the flexibility of their applications in many manufacturing-related scenarios where space is restricted.
Another disadvantage of strain gauge-based load measurement is the difficulty in attaching the strain gauge sensors to an object or component to be measured. For proper functioning, the attachment of strain gauge sensors to the mechanical structure requires the use of special glues and meticulous, time-consuming manual processing to ensure proper bonding of the strain gauge sensors. Such glues have a limited fatigue life, and are sensitive to temperature variations. In addition, strain gauges themselves are sensitive to environmental influences (such as temperature, humidity, etc.), and therefore, require the use of sophisticated compensation electronics to maintain accurate measurement. Such a requirement increases the cost and space needed.
These drawbacks make it impractical and very difficult for strain gauges to be effectively integrated into a realistic machine environment for on-line, in-process static load measurement, especially under space constraints.
Alternatively, piezoelectric sensing devices (which provide mechanical load to electrical charge transduction) have been commercially available for many years. The wide bandwidth and fast response of piezoelectric materials make piezoelectric sensors especially suited for a measuring load that varies at high frequencies. However, because of the inherent problem of charge leakage resulting and the subsequent loss of information due to the non-infinite insulation resistance of the piezoelectric sensors, it has been traditionally considered impossible to use piezoelectric sensors for pure static load measurement.
It is accordingly an objective of the present invention to provide static load sensing based on the piezoelectric effect. It is yet another objective of the present invention to provide a measurement technique that utilizes a piezoelectric sensor for static load measurement purposes. Such an invention can include a feedback technique that compensates for the signal loss due to charge leakage, and ensure measurement accuracy. It is yet another objective to provide a “zero-line” as the reference base for absolute load reading.
The apparatus of the present invention can be of durable construction, requiring little or no maintenance over its operating lifetime. In order to enhance market appeal, such an apparatus should also be of inexpensive construction to afford broadest possible application. Finally, it is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.